The present invention relates to a friction feeder for feeding sheets of paper from a stack. More particularly it relates to a feeder which employs a flexible friction belt. Typically, these belts are mounted on at least two pulleys which are spaced apart and define the initial advancement path of the sheet. Some typical examples of the many sheet feeding apparatus employing endless friction belts are disclosed in U.S. Pat. Nos. 3,485,489; 3,734,790; 3,768,803; 3,941,373; 3,957,366; and 4,025,187.
The geometric relationship and normal forces between the feed belt and head of the paper stack are crucial to achieving good feeder performance. While prior art feeders such as those illustrated in the aforementioned patents have been somewhat successful and found application in combination with present day high speed copy machines, their effectiveness is restricted by the mechanics upon which they are based. While in some instances elaborate mechanisms are employed to bias and hold the feed belt in contact with the paper stack, often these same mechanisms limit the degree of freedom with which the belt may move and, in turn, limit the belt's ability to adjust and conform to changes in the paper and stack condition. For instance, the mere fact that the belt is mounted for rotation on two pulleys itself limits the freedom of movement of the belt to one axis. Furthermore, contact between the belt and the stack is governed and often intentionally limited to a small area on the stack in prior art feeders by the orientation of the feeder belt and pulleys and their location with respect to the stack. When the movement of the belt is restricted to one axis, it is unable to conform to the stack and paper condition and it is over-sensitive to changes in the stack height. As a result, there is an undesirable misfeed frequency.
These feeders are typified in Stange, U.S. Pat. No. 3,768,803 (mentioned above). Stange features an arrangement where the feed belt removes the uppermost sheet of paper from the stack and (in cooperation with a retard means overwhich it rides) forms a queuing throat where the sheets are separated and aligned. Stange limits the contact between the feed belt and the stack to a point at the stack edge to minimize intersheet coupling and so that the belt acts on the sheet for the longest distance. Because this arrangement relies upon such minimal contact between the stack and the sheet and because the motion of the feed belt itself is limited to its path about the pulleys on which it is mounted, the Stange arrangement is relatively sensitive to changes in the nature and quality of the paper as well as changes in the stack height. Stange, U.S. Pat. No. 3,941,373 represents an analogous arrangement.
Lindquist, U.S. Pat. No. 3,485,489 also teaches a belt feeder in which the belt is positioned over a pair of pulleys to form a so-called tractor which is positioned to move a sheet of paper from the stack to an adjacent horizontal surface. In this embodiment the tractor can pivot from a position of engagement with the stack to a position of disengagement, but the freedom of movement of the belt and its contact with the stack are limited.
Park, U.S. Pat. No. 3,734,490 illustrates a feeding mechanism in which the belt is mounted on a pair of pulleys and traverses an idler wheel positioned between the pulleys inside the belt's path. The pulleys upon which the belt is mounted are spaced from the stack, but the idler wheel is movable up and down to force the belt into engagement with the top document in the stack. Lindquist and Parker illustrate a further drawback of prior friction belt feeder. That is, the belt is often biased against the paper stock using elaborate mechanisms which are not only expensive but subject to their own mechanical failures.
Bottom feeding devices, i.e., feeders adapted to remove a sheet from the bottom of a stack, sometimes take greater advantage of the flexibility in the belt and its ability to conform to the paper condition. Some examples are Strobel, U.S. Pat. No. 3,934,869 and Kyhl, U.S. Pat. No. 3,988,017. These arrangements, however, have their own disadvantages, in particular, as the amount of paper in the stack decreases, the normal force between the stack and the belt changes. Thus, while frictional feed belts have been used extensively in paper feeders, there is still a need for improvement. In particular, there is a need for a feeder system which is more responsive and less sensitive to changes in the stack height and the paper condition.